The present invention relates to a method of treating an aqueous latex of a synthetic polymer which contains unreacted residual monomer from the polymerisation reaction used for the production of the polymer.
The production of a latex of a synthetic polymer is conventionally carried out by polymerising the constituent monomer or monomers for the polymer using an aqueous emulsion or micro suspension process. The latex, after polymerisation, invariably contains a significant quantity of residual monomer. In cases where the monomer is a low boiling liquid (e.g. vinyl chloride) and the polymerisation has been carried out under the autogenous monomer pressure, much of the residual monomer in the reaction vessel at the end of the polymerisation may be removed by venting (via e.g. a monomer trap). Nevertheless, even in these cases, the resulting latex will still contain an appreciable quantity of residual monomer after venting, e.g. dissolved in the aqueous phase and/or absorbed by the polymer particles.
It is often desirable to remove virtually all of the residual monomer from the polymer, particularly if the quality of the polymer is improved thereby, or if the monomer is expensive and merits recovery, or if the presence of even very small quantities of monomer in the polymer represents a hazard (e.g. a toxicity or an explosion hazard). It is therefore desirable to significantly reduce the residual monomer context of the latex since this places a lower burden on the equipment employed in subsequent work-up stages for removing and trapping the last traces of monomer and may also allow the latex to be stored with a greater degree of safety.
It is known to separate residual monomer from an aqueous polymer latex or slurry by sparging the polymer dispersion with a hot inert vapour or gas such as steam, wherein steam is bubbled through the dispersion by injecting steam into the dispersion and/or causing the dispersion to boil; the efficiency of the sparging process may be improved by operation at an elevated temperature and under reduced pressure. However, this technique can be disadvantageous when applied to polymer latices since the treatment of the latex in bulk for a prolonged period of time at a high temperature may impair its stability which can lead to coagulation problems. Furthermore, application of reduced pressure, which would allow a shorter sparging period at high temperatures, often causes unacceptable foaming difficulties. In view of this, the sparging technique when applied to latices must often be operated at rather low temperatures which may not always provide effective monomer separation.
It has been proposed to overcome these difficulties by using a continuous process wherein a latex is applied at the top of a tower containing a vertical tubular column (or a plurality of spaced vertical tubular columns) so that the latex overflows into the column and descends down it as a thin film on the inner wall thereof while being contacted at an elevated temperature with a co- or counter-current stream of an inert gas such as air or nitrogen. This technique is nevertheless not entirely satisfactory if applied in respect of latices containing large quantities of residual monomer (particularly if that monomer is a low boiling substance such as vinyl chloride) since extensive foaming may occur within the column, causing flooding of the descending latex.